As is known, the greater demand for mobility, the growing urbanization of the territory, pressing technological development, the increasing value of urban land and the investment capacity of institutions have resulted in considerable development as regards the construction of mass transport infrastructures, of the road, motorway, railway and underground railway type.
Such infrastructures mainly occupy underground space which offers areas available for sustainable development of infrastructures.
This development has also been strongly facilitated by the exponential technological development of excavating equipment which makes it possible to do works once prohibitive, in reliable times and at reliable costs, while respecting and totally safeguarding safety, the territory and pre-existences.
This has given rise to and permitted the building, planning and design of major infrastructures in terms of complexity and efficiency, aimed at minimizing risks and maximizing user safety.
Furthermore, the quest for continuous improvements to transport infrastructures, not only in terms of operating capacity and management but also of the safety of users during operation, involves further developments in technological installations and fittings which increase the value of the work.
In this respect, the fact is underlined that underground transport infrastructures stretching for long distances (over 2,000 m) base their safety concept during operation on the double pipe configuration, each of which unidirectional, connected to transversal passages called “bypasses” or “cross passages”.
In case of an accident, fire or other catastrophic event involving one of the two pipes, the transversal passages make it possible to place all the environments of the tunnel in communication to use the other pipe as a safe place and/or escape route.
The construction of the bypass tunnel is usually carried out after the two main pipes, which are excavated using special mechanical boring machines to support the balanced front, have been made.
This equipment makes excavating the main pipes of the tunnels efficient and safe with reliable and low building costs.
Building bypass tunnels on the other hand has no comparable excavation alternative using mechanized systems similar to the boring machines for excavating the main tunnels.
The building of the “cross passages”, in the majority of installations, requires the completion of excavations using traditional method, i.e., removing the earth with appropriate mechanical means (excavators, rippers, bucket excavators, . . . ) following a preliminary treatment of the soil so as to improve its mechanical characteristics.
The excavation operations are performed following a defined time sequence which envisages:                excavation of the two main pipes generally done using tunnel boring machines wherein the excavation and lining of the tunnels is done in an automated way. The lining consists of reinforced concrete rings consisting of a certain number of precast segments assembled on site;        carrying out of pre-consolidation jobs on the outline of the future bypass section for the purpose of improving the mechanical characteristics of the material to be excavated. Such jobs must be performed from one or both main tunnels, often in the presence of very tight spaces to accommodate the equipment needed to perform such jobs, or, if possible, working from the surface;        installation of a structure to support the segment lining, generally consisting of metal profile sections, which must in part be demolished to create the opening from where to approach the bypass excavation;        after the demolition of part of the tunnel lining, bypass excavation, which is performed using excavators and other machines for recesses of about one meter followed by the installation of the first-phase lining, generally consisting of shotcrete reinforced with metal profile sections, known as centring;        after completion of the bypass excavation with the demolition of the arrival tunnel lining, installation of the bypass impermeabilization system, which is applied directly on the first-phase lining and consists of membranes made of sheets of plastic material or sprayed, subsequently lined with an on-site and usually reinforced concrete casting;        preparation of finishes and plant engineering systems inside the bypass which permit starting its operation.        
Such infrastructures are habitually used in the following two ambits:                building of crossing pass tunnels or underpasses of more or less important morphological elevations more than 2,000 m long, prevalently performed in rock masses with discreet mechanical characteristics in generally not very urbanized contexts with generally rather limited impacts on the context. The sensitivity of the building ambit as regards the construction of the installation is rather low and building complexity is not generally tied to the characteristics of the geological-geotechnical-hydrogeological ambit but rather to the unknown factors intrinsically involved;        tunnels connected to the underpassing of urban contexts and infrastructures tied to mass transport metropolitan networks, which are generally performed in a strongly urbanized context sensitive and susceptible to interferences with excavation operations. They consist in rather complex installations mainly because of the concurrence of two dominating factors, i.e., the geological-geotechnical-hydrogeological context and the sensitivity of the environment to the excavation. These installations are generally made inside loose soils with poor mechanical characteristics, often located below the level of the water table with reduced cover between the tunnel crown and the ground level, and sometimes also with the presence of (natural) gas.        
It is therefore easy to appreciate that the construction of underground transport infrastructures complete with bypass tunnels is a very complex and problematic activity, in particular when performed in urban and metropolitan contexts, and the need is strongly felt to find cutting edge technology and innovative solutions such as to allow building the above installation parts in an efficient, safe and easily repeatable way, so as to maximize the benefits and minimize risks, above all where reference is made to the bypass tunnel building phase.
The excavation and bypass building method most widely used to date in fact certainly does not achieve the level of mechanization and industrialization applicable for the excavation of the main tunnels.
The procedures used, above all in the case of work excavations and consequently cross passages involving loose soils, underneath the water table, in particularly sensitive urban contexts, are rather complex, localizing in the areas of reference particularly difficult soil consolidation and impermeabilization jobs (freezing, injections by means of concrete and/or chemical mixes) in order to allow excavating bypass tunnels in conditions of safety for the workers and the urban context.
The above jobs involve a number of difficulties/critical situations, listed below:                particularly restricted work environment for carrying out the excavation and consolidation jobs;        high sensitivity and dependency of the excavation operations and cable stability on the success of the consolidation jobs;        risks relating to the imperfect success of the cable impermeabilization jobs due to the effect of the injection jobs;        low level of industrialization of the building processes which potentially reduce the level of safety and quality of the building process.        